but any liquid water that had condensed in the tire surely would vaporize and expand hundreds of times
Thanks Chu Chu - that is the essence of numerous arguments I've had with fellow racers. There is essentially no difference between DRY air and DRY nitrogen. If you pick up a tank of compressed gas from the local compressed gas vendor, it is almost certainly 'dry' since that is part of their bottling process - nitrogen is popular because it's readily available as a byproduct of producing bottled O2. And even when you throw H20 into the mix, it only comes into play
if there is enough moisture that it changes state - in which case of course the effect can be huge (which is rarely the case). The funny part is I've watched guys spray 'soapy water' all over the bead and rim to help seat the bead when mounting tires, then brag about how using nitrogen will prevent the tire pressure from changing as the tires heated.
Oh, and unlike aircraft tires which are inflated to around 200 psi, we typically ran around one bar pressure - in which case whatever was inside the tire prior to beading was a significant percentage of the total.
Peekay, I talked to a chemist buddy, and at least according to him, unless the Oxygen was "ionized" (missing electrons), the O2 molecule would 'act' larger than the N2 molecule when it came to passing through other materials. N2 and O2 are basically symmetrical molecules so their molecular weight does closely relate to their size (H2O, being highly asymmetric, does act much differently than what it's molecular weight would suggest).